With the increased penetration of wind generation into the grid, various ancillary services are being requested to such generators, such as voltage and frequency control, to ensure their proper integration into the grid.
Thus, active and reactive power controls have been developed to help stabilize the frequency and voltage, respectively, in normal operation, understanding this operating range to be that specified in the corresponding grid code.
Conventionally, two different strategies have been used to control the voltage of the wind turbines in normal operation.
The first strategy has been to incorporate wind farm controllers which, based on the voltage measured at the wind farm connection point, send reactive power or power factor set points to the wind turbines. Examples of this type of control can be found in patent application EP1433238 and part of what is disclosed in U.S. Pat. No. 7,166,928B2.
The disadvantage of this solution is that to achieve a rapid response, a sophisticated communication network connecting the wind farm controller to the wind turbines is required.
Another disadvantage of this system is that, as the terminal voltage of the wind turbines is not controlled, the reactive power set point requested from the wind turbine may change it, possibly exceeding the allowed voltage range and causing the shutdown of the machine.
The second strategy has been to incorporate terminal voltage controllers in the wind turbines. An example of this type of control can be found in U.S. Pat. No. 6,965,174 B2.
This type of control provides a rapid response and requires no additional substation control equipment, since the wind turbines always incorporate controllers and measurements of the connection terminal voltage.
However, the disadvantage is that it only controls the local voltage, whose value is not relevant if kept within specified operating ranges. On the other hand, the voltage at the farm connection point is not controlled and therefore may be subject to variations.
Solutions whereby local controls are implemented integrated with central controls at farm level, thereby improving the response of the previous controls, are found in the following patent applications EP1512869A1, WO2006037576A1 and WO2006120033A2.
In addition, in recent years other ancillary services are being demanded in view of grid events, such as the generation of reactive power in a voltage dip, whereas previously the only requirement was to remain connected to the grid during the failure.
In the same way as in normal operation, several controls have been developed for the generation of reactive power in the event of a voltage dip in order to contribute to the restoration of voltage, as shown in the patent application US2007/0273155 A1.
The disadvantage of such references is that to comply with all grid requirements, they have specific controls for normal operation and other specific controls for different grid events such as the aforementioned voltage dips, so that at the time of a failure occurring, the controls associated with operating in normal mode are deactivated to enable the fault control, thus producing discontinuities in the control.
Similarly, upon restoring the voltage, the type of control must change again and the variables of the different controllers have to be recalculated and adapted to new grid conditions, through a series of complex calculations. This series of discontinuities in the control generate a response of the wind turbine in terms of the grid integration that may be improved and complex controller initialization algorithms are required for proper operation.
On the other hand, the wind turbines have to stay connected to the grid during voltage dips for more or less time and for different depths depending on the applying grid code in each case.
The electric power that can be evacuated to the grid decreases proportionally to the depth of the dip. If the power captured from the wind remains unchanged and the electrical power that can be evacuated is less than the former, there is an acceleration of the rotor which can lead the machine to an emergency stop due to excessive speed, thereby failing to comply with the aforementioned grid codes.
U.S. Pat. No. 6,921,985 discloses a blade pitch control in response to the transition between a first mode of operation and a second mode of operation, given such transition by the event of a voltage dip in the grid. The aforementioned patent identifies voltage dip depth thresholds based on which the mode of operation is adjusted.
Patent application WO2008/031433 discloses a method for controlling the blade pitch in the transition between the voltage dip and normal operation, whereby a variable of the power supply (e.g. voltage) is measured and translated into a variable that takes a value in a normal situation and another different value in a voltage dip.
The abovementioned background documents limit the capture solely based on the detection or not of a dip, which on the one hand can lead to a limitation in situations where it is not necessary, or not limit in others where it is necessary, depending both on the voltage range in which “voltage dip mode of operation” is detected and on the wind power available.
U.S. Pat. No. 6,906,431 B2 discloses a method of controlling a wind farm whereby constant apparent power is generated.
The relationship is known between apparent power (S), voltage (V), current (i), active power (P) and reactive power (Q) according to the well-known expression:S=v*i=√{square root over (P2+Q2)}
Patent application WO2005031160A2 explains the concept of apparent power available of an electrical unit, which depends on the grid voltage and the maximum current that the equipment can withstand. Said application discloses a method for voltage dips whereby the reactive current set point (in quadrature with the voltage) is limited in terms of the maximum available current and the active current (in phase with the voltage).
On the other hand, “Lars Helle” published in his doctoral thesis (Apr. 10, 2007): “Modeling and comparison of power converters for doubly fed induction generators in wind turbines” the development of a tool suitable for comparing different power converter topologies for use in a wind turbine application based on the doubly-fed induction generator. The main focus in this thesis is to establish a simple, fast and accurate simulation tool for evaluating different power converter topologies for use in a wind turbine based on the doubly-fed induction generator. The objective is to be able to compare the turbine efficiency when using the different power converter topologies. Specifically, the report treats four power converter topologies, namely the back-to-back two level voltage source converter, the matrix converter, the back-to-back diode clamped three-level voltage source converter and the back-to-back transistor clamped three-level voltage source converter. It also provides a model for the wind turbine blade, given that the power captured from the wind depends on the blade design, the pitch angle and the tip speed ratio, and assuming that the wind turbine tracks the optimum pitch angle as long as the generated wind turbine power is below the nominal power of the system.
Variable-speed constant-frequency generating systems are used in wind power, hydroelectric power, aerospace and naval power generation applications to enhance efficiency and reduce friction. In these applications, a candidate is the slip power recovery system comprising a doubly excited induction machine or doubly excited brushless reluctance machine and PWM power converters with a DC link. In the article published by Yifan Tang and Longya Xu [vol. 10, issue 4, July 1995] in IEEE Transactions on Power Electronics, a flexible active and reactive power control strategy is developed, such that the optimal torque-speed profile of the turbine can be followed and overall reactive power can be controlled, while the machine copper losses have been minimized. At the same time, harmonics injected into the power network have also been minimized. In this manner, the system can function as both an efficient power generator and a flexible reactive power compensator.